It has been well known that after a silver halide photographic light-sensitive material is exposed to light, an oxidized aromatic primary amine developing agent reacts with dye forming couplers to form color images. In this process, color reproduction by a conventional substractive process is used to form color images composed of cyan, magenta, and yellow dyes, which are the complementary colors of red, green, and blue, respectively. The reaction of the couplers with the oxidation product of the color developing agent is carried out at an active site (usually referred to as the coupling position). The coupler having a hydrogen atom as a substituent at this active site is a 4-equivalent coupler, i.e., a coupler which stoichiometrically requires 4 moles of silver halide having a development nucleus as an oxidizing agent for forming 1 mol of dye.
On the other hand, a coupler having a group releasable in a state of an anion (which herein refers to "coupling-off group") as a substituent at the active site is a 2-equivalent coupler, i.e., a coupler which requires only 2 mols of silver halide having a development nucleus for forming 1 mol of dye. Accordingly, the processing time for the light-sensitive materials can be shortened, and the sharpness of the formed color images is improved when the 2-equivalent coupler is used, as compared with using the 4-equivalent coupler, because it is possible to reduce the amount of silver halide in the light-sensitive layer and to thin the thickness of the layer. In 2-equivalent couplers, it is possible to change the coupling activity to the color developing agent by changing the property of coupling-off group.
Furthermore, a 2-equivalent coupler which has the effect of restraining development by means of the released coupling-off product is called development inhibitor releasing coupler (DIR coupler), which restrains the development in proportion to an amount of development silver formed. Accordingly, it shows effects such as fine granulation of image, control of gradation, improvement of color reproduction, etc. Further, it can be utilized for a diffusion transfer process by utilizing its function to adjacent layers.
Also, when a diffusible dye portion is substituted as the coupling-off group at the active site of the 2-equivalent coupler, it is possible to use the coupler in a diffusion transfer process for forming diffusible dye images on an image receiving layer by utilizing a dye released. This kind of coupler is usually referred to as a diffusible dye releasing (DDR) coupler.
Additionally, certain kinds of colored 2-equivalent couplers shows a masking effect for color correction of dye images. These kinds of coupler are referred to as colored 2-equivalent coupler.
As described above, it is possible to provide various functions in 2-equivalent couplers by changing the coupling-off group.
Silver halide has been widely used in the photographic field, because it has the highest sensitivity of known light-sensitive materials. However, all of silver halides are not used sufficiently as light-sensitive elements, and it causes lowering of sensitivity of the silver halide photographic light-sensitive materials, deterioration of granularity deterioration of sharpness, etc. This is believed to be ascribable to the uneven development rate of each emulsion grain one another and in consequence existence of emulsion grains of retarded development, because of various causes such as differences of sensitive properties and development properties of silver halide emulsions, flickering of light at exposure or uneven development, etc.
In order to accelerate the development of emulsion grains exhibiting retarded development, it has been proposed hitherto to add various kinds of development accelerators such as hydrazine compounds, amine compounds, 1-phenylpyrazoline-3-one derivatives, hydroquinone derivatives, hydrogen peroxides, etc., to an emulsion layer or a developing solution, particularly, for black-and-white light-sensitive materials. However, they are not practically used because they frequently cause increases of fog or deterioration of granularity.
The present invention relates to couplers which release a diffusible development accelerator as the coupling-off group. These are referred to as development accelerator releasing (DAR) couplers. The conception of such development accelerator releasing (DAR) couplers have been disclosed in U.S. Pat. Nos. 3,214,337 and 3,253,924 and Japanese Patent Application (OPI) No. 17437/76 std. OPI def. (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). All of these disclosed couplers release a thiocyanic acid ion to accelerate the development by means of a dissolution physical development function. However, because many conventionally used emulsions are surface development type emulsions, the granularity resulting is inferior, because the increased rate of the number of development initiation points by the dissolution physical development is small and rather the development is accelerated by enlarging development initiation points.